Vehicle safety standards have evolved in an effort to provide safer passenger cars. A vehicle safety standard for roof crush resistance is FMVSS No. 216. The loading requirements of FMVSS No. 216 have been increased with the objective of providing greater protection for passengers in vehicle rollover events. The FMVSS standard will require that all vehicles meet a roof strength requirement of 3.0 times the vehicle weight. This requirement increase poses a significant challenge to all vehicle manufacturers. Increased roof strength requirements must be met while also achieving increased fuel economy that may demand lighter weight structures in the overall vehicle.
Large passenger vehicles often have three or more sets of vertical pillars supporting the roof structure. Pillars are typically referred to from front to rear as A, B, and C-pillars. Some vehicles also employ a fourth, D-pillar. In contrast, some small vehicles with only one row of doors have only two pillars. Vehicles with front and rear side doors generally have a middle B-pillar. The B-pillar defines the separation between separate front and rear door openings. Existing roof structures rely substantially on a mid-vehicle vertical B-pillar to sustain vertical roof crash loads. The size of the B-pillar required to meet roof crush requirements may obstruct access to the vehicle by occupants. It generally restricts the space available for door openings, and therefore the ease of entry and exit by the occupants of the vehicle. The B-pillar also limits the size of objects that are capable of being loaded through the door openings. The B-pillar may also obstruct the driver's field of view. The B-pillar also presents vehicle styling limitations, since its placement is often dictated by functional requirements. Although the B-pillar has been eliminated in certain vehicle types, such as light trucks, offering several styling and space advantages, meeting increasing roof strength requirements remains problematic.
One known vehicle roof system enabling elimination of the B-pillar transfers vertical roof loads onto an enhanced rear structure of the vehicle. An aspect of this system is the addition of upper cap reinforcement directly to assume roof crush loads applied at the front of the vehicle roof during testing. A specific load transfer mechanism is incorporated wherein vertical roof crush force applied near the front of the vehicle is transferred into both a torsional load upon a rear header of the vehicle and a bending moment upon the C-pillar of the vehicle. Although this system enabled elimination of the B-pillar, it does so by increasing the mass and complexity of the upper rear structure of the vehicle roof. For example, it requires several discrete elongated structural elements to form the C-pillar and the adjacent side and rear frames.
Conventional body frames are typically fabricated as multiple stamped sheet metal parts that are generally spot welded together. It is possible to improve the strength of conventional body frames by forming the sheet metal parts from high grade material such as dual phase and boron steels. Body frames may also be made stronger by using thicker gauge sheet metal components. However, the use of high strength alloys and thicker sheet metal may increase the weight of the vehicle and also increase the cost to manufacture the body frame. Even with the use of thicker alloy components, the roof portions of conventional design body frames may not always meet stringent test requirements for roof crush performance.
Although stamped members have been used in vehicle body structures for years, hydroformed components or members may be used in vehicles. Hydroforming is a cost effective way of shaping malleable metals into lightweight, structurally stiff and strong elements. One of the largest applications for hydroforming is the automotive industry, which makes use of complex shapes possible by hydroforming to produce stronger, lighter and more rigid unibody structures for vehicles.
Hydroforming allows complex shapes to be formed, which would be difficult to manufacture with standard solid die stamping. Furthermore, hydroformed parts can often be made with a higher stiffness to weight ratio and at a lower per unit cost than traditional stamped or stamped and welded parts.
Another known vehicle roof system configuration employs a roof rail integral with an A-pillar and a support pillar. The structure further includes a cross member. The A-pillar includes an inner surface, an outer surface, and a wall there between. The roof rail extends downwardly at a front end of the roof rail and extends downwardly at a rear end of the roof rail. The roof rail is integral to a one piece hollow A-pillar at the front end of the roof rail. The support pillar also includes an inner surface, an outer surface, and a wall there between. The support pillar also includes a tubular lower section that extends upwardly from the rocker. The upper section of the support pillar is integral to the rear end of the roof rail. Although providing certain advantages, this system can prove difficult to fabricate.